The present invention generally relates to intravascular stents and more particularly pertains to improvements thereto that provide for increased coverage and greater expansion ratios without a compromise in strength.
Stents or expandable grafts are implanted in a variety of body lumens in an effort to maintain their patency. These devices are typically intraluminally implanted by use of a catheter which is inserted at an easily accessible location and then advanced to the deployment site. The stent is initially in a radially compressed or collapsed state to enable it to be maneuvered through the lumen. Once in position, the stent is deployed which, depending upon its configuration, is achieved either automatically or actively by the inflation of a balloon about which the stent is carried on the catheter.
As stents are normally employed to hold open an otherwise blocked, constricted or occluded lumen, a stent must exhibit sufficient radial or hoop strength in its expanded state to effectively counter the anticipated forces. Not only is it advantageous to distribute such loads over as much of the stent as possible but it also is most beneficial to distribute the load over as much lumen wall as possible. This will help minimize injury to the vessel wall. Also by minimizing the gaps between stent struts it is possible to prevent prolapse of the plaque between the struts into the lumen. As a consequence, it is desirable to maximize the coverage of the lumen wall by creating uniform, small gaps between the stent struts. It is, however, simultaneously necessary for the stent to be as small and compact as possible in its collapsed state in order to facilitate its advancement through the lumen. As large an expansion ratio as possible is therefore most desirable.
A number of very different approaches have been previously devised in an effort to address these various requirements. A popular approach calls for the stent to be constructed wholly of wire. The wire is bent, woven and/or coiled to define a generally cylindrical structure in a configuration that has the ability to undergo radial expansion. The use of wire has a number of disadvantages associated therewith including for example, a substantially constant cross-section which may cause greater or lesser than an ideal amount of material to be concentrated at certain locations along the stent. Additionally, wire has limitations with respect to the shapes it can be formed into thus limiting the expansion ratio, coverage area and strength that can ultimately be attained therewith. The welding of adjoining sections of wire together has also been previously employed to increase strength albeit with a substantial increase in manufacturing costs.
As an alternative to wire-based structures, stents have been constructed from tube stock. By selectively removing material from such tubular starting material, a desired degree of flexibility and expandability can be imparted to the structure. Chemical etching techniques as well as laser-cutting processes are utilized to remove material from the tube. Laser etching provides for a high degree of precision and accuracy with which very well defined patterns of material can be removed from the tube to conversely leave very precisely and accurately defined patterns of material in tact. The performance parameters of a function of the pattern in which material is removed form the tube stock. The selection of a particular pattern has a profound effect on the coverage area, expansion ratio and strength of the resulting stent.
While the tube-based stents offer many advantages over the wire-based designs, it is nonetheless desirable to improve upon such designs in an effort to further increase coverage area and expansion ratios while maintaining strength.